Luminescent ceramic for a light emitting device

ABSTRACT

A semiconductor light emitting device comprising a light emitting layer disposed between an n-type region and a p-type region is combined with a ceramic layer which is disposed in a path of light emitted by the light emitting layer. The ceramic layer is composed of or includes a wavelength converting material such as a phosphor. Luminescent ceramic layers according to embodiments of the invention may be more robust and less sensitive to temperature than prior art phosphor layers. In addition, luminescent ceramics may exhibit less scattering and may therefore increase the conversion efficiency over prior art phosphor layers.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/149,454, filed May 9, 2016, and titled “Luminescent Ceramic for aLight Emitting Device”, which is a division of U.S. patent applicationSer. No. 12/034,588, filed Feb. 20, 2008, now issued as U.S. Pat. No.9,359,260 on Jun. 7, 2016, and which is a division of U.S. patentapplication Ser. No. 10/861,172, filed on Jun. 3, 2004, now issued asU.S. Pat. No. 7,361,938 on Apr. 22, 2008. U.S. patent application Ser.Nos. 15/149,454, 12/034,588, and 10/861,172 are incorporated herein.

BACKGROUND Field of Invention

The present invention relates to wavelength converted semiconductorlight emitting devices.

Description of Related Art

Light emitting diodes (LEDs) are well-known solid state devices that cangenerate light having a peak wavelength in a specific region of thelight spectrum. LEDs are typically used as illuminators, indicators anddisplays. Traditionally, the most efficient LEDs emit light having apeak wavelength in the red region of the light spectrum, i.e., redlight. However, III-nitride LEDs have been developed that canefficiently emit light having a peak wavelength in the UV to greenregion of the spectrum. III-nitride LEDs can provide significantlybrighter output light than traditional LEDs.

In addition, since light from III-nitride devices generally has ashorter wavelength than red light, the light generated by theIII-nitride LEDs can be readily converted to produce light having alonger wavelength. It is well known in the art that light having a firstpeak wavelength (the “primary light”) can be converted into light havinga longer peak wavelength (the “secondary light”) using a process knownas luminescence/fluorescence. The fluorescent process involves absorbingthe primary light by a wavelength-converting material such as aphosphor, exciting the luminescent centers of the phosphor material,which emit the secondary light. The peak wavelength of the secondarylight will depend on the phosphor material. The type of phosphormaterial can be chosen to yield secondary light having a particular peakwavelength.

With reference to FIG. 1, a prior art phosphor LED 10 described in U.S.Pat. No. 6,351,069 is shown. The LED 10 includes a III-nitride die 12that generates blue primary light when activated. The III-nitride die 12is positioned on a reflector cup lead frame 14 and is electricallycoupled to leads 16 and 18. The leads 16 and 18 conduct electrical powerto the III-nitride die 12. The III-nitride die 12 is covered by a layer20, often a transparent resin, that includes wavelength-convertingmaterial 22. The type of wavelength-converting material utilized to formthe layer 20 can vary, depending upon the desired spectral distributionof the secondary light that will be generated by the fluorescentmaterial 22. The III-nitride die 12 and the fluorescent layer 20 areencapsulated by a lens 24. The lens 24 is typically made of atransparent epoxy or silicone.

In operation, electrical power is supplied to the III-nitride die 12 toactivate the die. When activated, die 12 emits the primary light awayfrom the top surface of the die. A portion of the emitted primary lightis absorbed by the wavelength-converting material 22 in the layer 20.The wavelength-converting material 22 then emits secondary light, i.e.,the converted light having a longer peak wavelength, in response toabsorption of the primary light. The remaining unabsorbed portion of theemitted primary light is transmitted through the wavelength-convertinglayer, along with the secondary light. The lens 24 directs theunabsorbed primary light and the secondary light in a general directionindicated by arrow 26 as output light. Thus, the output light is acomposite light that is composed of the primary light emitted from die12 and the secondary light emitted from the wavelength-converting layer20. The wavelength-converting material may also be configured such thatvery little or none of the primary light escapes the device, as in thecase of a die that emits UV primary light combined with one or morewavelength-converting materials that emit visible secondary light.

As III-nitride LEDs are operated at higher power and higher temperature,the transparency of the organic encapsulants used in layer 20 tend todegrade, undesirably reducing the light extraction efficiency of thedevice and potentially undesirably altering the appearance of the lightemitted from the device. Several alternative configurations ofwavelength-converting materials have been proposed, such as growth ofLED devices on single crystal luminescent substrates as described inU.S. Pat. No. 6,630,691, thin film phosphor layers as described in U.S.Pat. No. 6,696,703, and conformal layers deposited by electrophoreticdeposition as described in U.S. Pat. No. 6,576,488 or stenciling asdescribed in U.S. Pat. No. 6,650,044. However, one major disadvantage ofprior solutions is the optical heterogeneity of the phosphor/encapsulantsystem, which leads to scattering, potentially causing losses inconversion efficiency.

SUMMARY

In accordance with embodiments of the invention, a semiconductor lightemitting device comprising a light emitting layer disposed between ann-type region and a p-type region is combined with a ceramic layer whichis disposed in a path of light emitted by the light emitting layer. Theceramic layer is composed of or includes a wavelength convertingmaterial such as a phosphor. Luminescent ceramic layers according toembodiments of the invention may be more robust and less sensitive totemperature than prior art phosphor layers. In addition, luminescentceramics may exhibit less scattering and may therefore increase theconversion efficiency over prior art phosphor layers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior art phosphor-converted semiconductor lightemitting device.

FIG. 2 illustrates a flip chip semiconductor light emitting deviceincluding a ceramic phosphor layer.

FIG. 3 illustrates a semiconductor light emitting device including abonded host substrate and a ceramic phosphor layer.

FIG. 4 illustrates an example of a doping profile in a ceramic phosphorlayer.

FIG. 5 illustrates a semiconductor light emitting device includingmultiple ceramic layers.

FIG. 6 illustrates a semiconductor light emitting device including ashaped ceramic phosphor layer.

FIG. 7 illustrates a semiconductor light emitting device including aceramic phosphor layer wider than the epitaxial layers in the device.

FIG. 8 illustrates a semiconductor light emitting device including aceramic phosphor layer and a heat extraction structure.

DETAILED DESCRIPTION

The above-mentioned devices with thin film or conformal phosphor layerscan be difficult to handle because the phosphor layers tend to befragile. In accordance with embodiments of the invention, wavelengthconverting materials such as phosphors are formed into ceramic slabs,referred to herein as “luminescent ceramics.” The ceramic slabs aregenerally self-supporting layers formed separately from thesemiconductor device, then attached to the finished semiconductor deviceor used as a growth substrate for the semiconductor device. The ceramiclayers may be translucent or transparent, which may reduce thescattering loss associated with non-transparent wavelength convertinglayers such as conformal layers. Luminescent ceramic layers may be morerobust than thin film or conformal phosphor layers. In addition, sinceluminescent ceramic layers are solid, it may be easier to make opticalcontact to additional optical elements such as lenses and secondaryoptics, which are also solid.

Examples of phosphors that may be formed into luminescent ceramic layersinclude aluminum garnet phosphors with the general formula(Lu_(1-x-y-a-b)Y_(x)Gd_(y))₃(Al_(1-z)Ga_(z))₅O₁₂:Ce_(a)Pr_(b) wherein0<x<1, 0<y<1, 0<z≤0.1, 0<a≤0.2 and 0<b≤0.1, such as Lu₃Al₅O₁₂:Ce³⁺ andY₃Al₅O₁₂:Ce³⁺ which emit light in the yellow-green range; and(Sr_(1-x-y)Ba_(x)Ca_(y))_(2-z)Si_(5-a)Al_(a)N_(8-a)O_(a):Eu_(z) ²⁺wherein 0≤a<5, 0<x≤1, 0≤y≤1, and 0<z≤1 such as Sr₂Si₅N₈:Eu²⁺, which emitlight in the red range. Suitable Y₃Al₅O₁₂:Ce³⁺ ceramic slabs may bepurchased from Baikowski International Corporation of Charlotte, N.C.Other green, yellow, and red emitting phosphors may also be suitable,including (Sr_(1-a-b)Ca_(b)Ba_(c))Si_(x)N_(y)O_(z):Eu_(a) ²⁺(a=0.002-0.2, b=0.0-0.25, c=0.0-0.25, x=1.5-2.5, y=1.5-2.5, z=1.5-2.5)including, for example, SrSi₂N₂O₂:Eu²⁺;(Sr_(1-u-v-x)Mg_(u)Ca_(v)Ba_(x))(Ga_(2-y-z)Al_(y)In_(z)S₄):Eu²⁺including, for example, SrGa₂S₄:Eu²⁺; Sr_(1-x)Ba_(x)SiO₄Eu²⁺; and(Ca_(1-x)Sr_(x))S:Eu²⁺ wherein 0<x≤1 including, for example, CaS:Eu²⁺and SrS:Eu²⁺.

A luminescent ceramic may be formed by heating a powder phosphor at highpressure until the surface of the phosphor particles begin to soften andmelt. The partially melted particles stick together to form a rigidagglomerate of particles. Unlike a thin film, which optically behaves asa single, large phosphor particle with no optical discontinuities, aluminescent ceramic behaves as tightly packed individual phosphorparticles, such that there are small optical discontinuities at theinterface between different phosphor particles. Thus, luminescentceramics are optically almost homogenous and have the same refractiveindex as the phosphor material forming the luminescent ceramic. Unlike aconformal phosphor layer or a phosphor layer disposed in a transparentmaterial such as a resin, a luminescent ceramic generally requires nobinder material (such as an organic resin or epoxy) other than thephosphor itself, such that there is very little space or material of adifferent refractive index between the individual phosphor particles. Asa result, a luminescent ceramic is transparent or translucent, unlike aconformal phosphor layer.

Luminescent ceramic layers may be attached to light emitting devices by,for example, wafer bonding, sintering, gluing with thin layers of knownorganic adhesives such as epoxy or silicone, gluing with high indexinorganic adhesives, and gluing with sol-gel glasses.

Examples of high index adhesives include high index optical glasses suchSchott glass SF59, Schott glass LaSF 3, Schott glass LaSF N18, andmixtures thereof. These glasses often have an index of refractiongreater than 1.8 and are available from Schott Glass TechnologiesIncorporated, of Duryea, Pa. Examples of other high index adhesivesinclude high index chalcogenide glass, such as (Ge,Sb,Ga)(S,Se)chalcogenide glasses, III-V semiconductors including but not limited toGaP, InGaP, GaAs, and GaN, II-VI semiconductors including but notlimited to ZnS, ZnSe, ZnTe, CdS, CdSe, and CdTe, group IV semiconductorsand compounds including but not limited to Si and Ge, organicsemiconductors, metal oxides including but not limited to tungstenoxide, titanium oxide, nickel oxide, zirconium oxide, indium tin oxide,and chromium oxide, metal fluorides including but not limited tomagnesium fluoride and calcium fluoride, metals including but notlimited to Zn, In, Mg, and Sn, yttrium aluminum garnet (YAG), phosphidecompounds, arsenide compounds, antimonide compounds, nitride compounds,high index organic compounds, and mixtures or alloys thereof. Gluingwith high index inorganic adhesives is described in more detail inapplication Ser. No. 09/660,317, filed Sep. 12, 2000, and Ser. No.09/880,204, filed Jun. 12, 2001, both of which are incorporated hereinby reference.

Gluing with sol-gel glasses is described in more detail in U.S. Pat. No.6,642,618, which is incorporated herein by reference. In embodimentswhere the luminescent ceramic is attached to the device by a sol-gelglass, one or more materials such as oxides of titanium, cerium, lead,gallium, bismuth, cadmium, zinc, barium, or aluminum may be included inthe SiO₂ sol-gel glass to increase the index of refraction of the glassin order to more closely match the index of the glass with the indicesof the luminescent ceramic and the light emitting device. For example, aY₃Al₅O₁₂:Ce³⁺ ceramic layer may have an index of refraction of betweenabout 1.75 and 1.8, and may be attached to a sapphire growth substrateof a semiconductor light emitting device, which sapphire substrate hasan index of refraction of about 1.8. It is desirable to match therefractive index of the adhesive to the refractive indices of theY₃Al₅O₁₂:Ce³⁺ ceramic layer and the sapphire growth substrate.

In some embodiments, a luminescent ceramic serves as a growth substratefor the semiconductor light emitting device. This is especiallyplausible with III-nitride light emitting layers such as InGaN, whichare able to be grown on a lattice-mismatched substrate (e.g., sapphireor SiC), resulting in high dislocation densities, but still exhibit highexternal quantum efficiency in LEDs. Thus, a semiconductor lightemitting device may be grown on a luminescent ceramic in a similarmanner. For example, using metal-organic chemical vapor-phase epitaxy oranother epitaxial technique, a III-nitride nucleation layer isdeposited, typically at low temperature (˜550° C.), directly on theluminescent ceramic substrate. Then, a thicker layer of GaN (‘buffer’layer) is deposited, typically at higher temperature, on the III-nitridenucleation layer and coalesced into a single crystal film. Increasingthe thickness of the buffer layer can reduce the total dislocationdensity and improve the layer quality. Finally, n-type and p-type layersare deposited, between which light emitting III-nitride active layersare included. The ability to withstand the III-nitride growthenvironment (e.g., temperatures greater than 1,000° C. and an NH₃environment) will govern the choice of luminescent ceramic as a growthsubstrate. Because the ceramics are poly-crystalline, and the resultingIII-nitride layers should be single crystal, special additional growthconsiderations may apply. For example, for the situation describedabove, it may be necessary to insert multiple low-temperatureinterlayers within the GaN buffer layer to ‘reset’ the GaN growth andavoid ceramic grain orientation effects from propagating into theIII-nitride device layers. These and other techniques are known in theart for growing on lattice-mismatched substrates. Suitable growthtechniques are described in, for example, U.S. Pat. No. 6,630,692 toGoetz et al., which is assigned to the assignee of the presentapplication and incorporated herein by reference.

Though the examples below refer to III-nitride light emitting diodes, itis to be understood that embodiments of the invention may extend toother light emitting devices, including devices of other materialssystems such as III-phosphide and III-arsenide, and other structuressuch as resonant cavity LEDs, laser diodes, and vertical cavity surfaceemitting lasers.

FIGS. 2 and 3 illustrate III-nitride devices including luminescentceramic layers. In the device of FIG. 2, an n-type region 42 is grownover a suitable growth substrate 40, followed by active region 43 andp-type region 44. Growth substrate 40 may be, for example, sapphire,SiC, GaN, or any other suitable growth substrate. Each of n-type region42, active region 43, and p-type region 44 may include multiple layersof different composition, thickness, and dopant concentration. Forexample, n-type region 42 and p-type region 44 may include contactlayers optimized for ohmic contact and cladding layers optimized tocontain carriers within active region 43. Active region 43 may include asingle light emitting layer, or may include multiple quantum well lightemitting layers separated by barrier layers.

In the device illustrated in FIG. 2, a portion of p-type region 44 andactive region 43 are etched away to reveal a portion of n-type region42. A p-contact 45 is formed on the remaining portion of p-type region44 and an n-contact 46 is formed on the exposed portion of n-contact 46.In the embodiment illustrated in FIG. 2, contacts 45 and 46 arereflective such that light is extracted from the device through the backside of substrate 40. Alternatively, contacts 45 and 46 may betransparent or formed in such a way that a large portion of the surfacesof p-type region 44 and n-type region 42 are left uncovered by contacts.In such devices, light may be extracted from the device through the topsurface of the epitaxial structure, the surface on which contacts 45 and46 are formed.

In the device illustrated in FIG. 3, the epitaxial layers are bonded toa host substrate 49 through p-contact 45. Additional layers tofacilitate bonding (not shown) may be included between p-type region 44and host 49. After the epitaxial layers are bonded to host 49, thegrowth substrate may be removed to expose a surface of n-type region 42.Contact to the p-side of the active region is provided through host 49.An n-contact 46 is formed on a portion of the exposed surface of n-typeregion 42. Light is extracted from the device through the top surface ofn-type region 42. Growth substrate removal is described in more detailin application Ser. No. 10/804,810, filed Mar. 19, 2004, titled“Photonic Crystal Light Emitting Device,” assigned to the assignee ofthe present invention, and incorporated herein by reference.

In the devices illustrated in FIGS. 2 and 3, a luminescent ceramic layer50 such as the ceramic layers described above, is attached (e.g., by aglass layer 51) to the surface of the device from which light isextracted; the back of substrate 40 in FIG. 2 and the top of n-typeregion 42 in FIG. 3. Ceramic layer 50 may be formed on or attached toany surface from which light is extracted from the device. For example,ceramic layer 50 may extend over the sides of the device illustrated inFIG. 2. FIG. 3 illustrates an optional filter 30, which allows lightfrom active region 43 to pass into ceramic layer 50, but reflects lightemitted by ceramic layer 50, such that light emitted by ceramic layer 50is inhibited from entering device 52, where it is likely to be absorbedand lost. Examples of suitable filters include dichroic filtersavailable from Unaxis Balzers Ltd. of Liechtenstein or Optical CoatingLaboratory, Inc. of Santa Rosa, Calif.

Luminescent ceramic layer 50 may include a single phosphor or multiplephosphors mixed together. In some embodiments, the amount of activatingdopant in the ceramic layer is graded. FIG. 4 illustrates an example ofa graded doping profile in a luminescent ceramic layer. The dashed linein FIG. 4 represents the surface of the device. The phosphor in theportion of the ceramic layer closest to the device surface has thehighest dopant concentration. As the distance from the device surfaceincreases, the dopant concentration in the phosphor decreases. Though alinear dopant profile with a region of constant dopant concentration isshown in FIG. 4, it is to be understood that the grading profile maytake any shape including, for example, a step-graded profile or a powerlaw profile, and may include multiple or no regions of constant dopantconcentration. In addition, in some embodiments it may be advantageousto reverse the grading profile, such that the region closest to thedevice surface has a small dopant concentration that increases as thedistance from the device surface increases. In some embodiments, theportion of the ceramic layer furthest from the device surface may notcontain any phosphor or any dopant, and may be shaped (as shown below)for light extraction.

In some embodiments, devices include multiple ceramic layers, as in thedevice illustrated in FIG. 5. Ceramic layer 50 a is attached (e.g., byglass layer 51) to device 52, which may be, for example, either of thedevices illustrated in FIGS. 2 and 3. Ceramic layer 50 b is attached toceramic layer 50 a. In some embodiments, one of the two ceramic layers50 a and 50 b contains all the wavelength converting materials used inthe device, and the other of the two ceramic layers is transparent andused as a spacer layer, if it is the ceramic layer adjacent to device52, or as a light extraction layer, if it is the ceramic layer furthestfrom device 52. In some embodiments, each of ceramic layers 50 a and 50b may contain a different phosphor or phosphors. Though two ceramiclayers are illustrated in FIG. 5, it is to be understood that devicesincluding more than two ceramic layers and/or more than two phosphorsare within the scope of the invention. The arrangement of the differentphosphors in ceramic layers 50 a and 50 b or ceramic layers 50 a and 50b themselves may be chosen to control the interaction between themultiple phosphors in a device, as described in application Ser. No.10/785,616 filed Feb. 23, 2004 and incorporated herein by reference.Though ceramic layers 50 a and 50 b are shown stacked over device 52 inFIG. 5, other arrangements are possible and within the scope of theinvention. In some embodiments, a device including one or more ceramiclayers may be combined with other wavelength converting layers, such asthe wavelength converting material shown in FIG. 1, or the thin films,conformal layers, and luminescent substrates described in the backgroundsection. Transparent ceramic layers that are not luminescent may be, forexample, the same host material as the luminescent ceramic layer,without the activating dopant.

An advantage of luminescent ceramic layers is the ability to mold,grind, machine, hot stamp or polish the ceramic layers into shapes thatare desirable, for example, for increased light extraction. Luminescentceramic layers generally have high refractive indices, for example 1.75to 1.8 for a Y₃Al₅O₁₂:Ce³⁺ ceramic layer. In order to avoid totalinternal reflection at the interface between the high index ceramiclayer and low index air, the ceramic layer may be shaped as illustratedin FIGS. 6 and 7. In the device illustrated in FIG. 6, the luminescentceramic layer 54 is shaped into a lens such as a dome lens. Lightextraction from the device may be further improved by texturing the topof the ceramic layer, either randomly or in, for example, a Fresnel lensshape, as illustrated in FIG. 7. In some embodiments the top of theceramic layer may be textured with a photonic crystal structure, such asa periodic lattice of holes formed in the ceramic. The shaped ceramiclayer may be smaller than or the same size as face of device 52 to whichit is attached (e.g., by glass layer 51) or it may be larger than theface of device 52 to which it is attached, as illustrated in FIGS. 6 and7. In devices such as FIG. 7, favorable light extraction is expected forshaped ceramic layers having a bottom length at least twice the lengthof the face of device 52 on which the ceramic layer is mounted. In someembodiments, the wavelength converting material is confined to theportion of the ceramic layer closest to the device 52. In otherembodiments, as illustrated in FIG. 7, the wavelength convertingmaterial is provided in a first ceramic layer 50 a, then attached to asecond, shaped, transparent ceramic layer 50 b.

In some embodiments, the surface of the top ceramic layer is roughenedto increase scattering necessary to mix the light, for example, in adevice where light from the light emitting device and one or morewavelength converting layers mixes to form white light. In otherembodiments, sufficient mixing may be accomplished by secondary opticssuch as a lens or light guide, as is known in the art.

A further advantage of luminescent ceramic layers is the favorablethermal properties of ceramics. A device including a luminescent ceramiclayer and a heat extraction structure is illustrated in FIG. 8. As inFIG. 7, FIG. 8 includes a transparent or luminescent ceramic layer 50 bthat is shaped for light extraction. An optional additional transparentor luminescent ceramic layer 50 a is disposed between layer 50 b anddevice 52. Device 52 is mounted on a submount 58, for example as a flipchip as illustrated in FIG. 2. Submount 58 and host substrate 49 of FIG.3, may be, for example, metals such as Cu foil, Mo, Cu/Mo, and Cu/W;semiconductors with metal contacts, such as Si with ohmic contacts andGaAs with ohmic contacts including, for example, one or more of Pd, Ge,Ti, Au, Ni, Ag; and ceramics such as compressed diamond. Layers 56 arethermally conductive materials that connect ceramic layer 50 b tosubmount 58, potentially reducing the temperature of luminescent ceramiclayer 50 a and/or 50 b, and thereby increasing light output. Materialsuitable for layers 56 include the submount material described above.The arrangement illustrated in FIG. 8 is particularly useful to extractheat from flip chip devices with conductive substrates, such as SiC.

EXAMPLE

An example of a cerium-doped yttrium aluminum garnet ceramic slabdiffusion-bonded to a sapphire substrate is given below.

Diffusion-bonded YAG-sapphire composites are advantageous because oftheir high mechanical strength and excellent optical quality. Accordingto the phase diagram yttria-alumina within the composition range Al₂O₃and 3 Y₂O₃ 5 Al₂O₃, no other phase exists except an eutecticum with 33%Al. Therefore, a sinterbonded YAG-sapphire composite has an averagerefractive index at the (eutectoidic) interface between YAG ceramic(n_(i)=1.84) and sapphire substrate (n_(i)=1.76) and thus a high qualityoptical contact can be obtained. In addition, because of the similarexpansion coefficients of YAG and sapphire (YAG: 6.9×10⁻⁶ K⁻¹, Al₂O₃:8.6×10⁻⁶ K⁻¹), sinterbonded wafers with low mechanical stress can beproduced.

A diffusion-bonded YAG:Ce ceramic-sapphire wafer may be formed asfollows:

a) Production of YAG:Ce ceramic: 40 g Y₂O₃ (99.998%), 32 g Al₂O₃(99.999%), and 3.44 g CeO₂ are milled with 1.5 kg high purity aluminaballs (2 mm diameter) in isopropanol on a roller bench for 12 hrs. Thedried precursor powder is then calcined at 1300° C. for two hours underCO atmosphere. The YAG powder obtained is then deagglomerated with aplanet ball mill (agate balls) under ethanol. The ceramic slurry is thenslip casted to obtain a ceramic green body after drying. The greenbodies are then sintered between graphite plates at 1700° C. for twohours.

b) Diffusion-bonding of a sapphire wafer and a YAG:Ce ceramic: Theground and polished sapphire and YAG wafers are diffusion bonded in auniaxial hot pressing apparatus (HUP). For this purpose, sapphire andYAG wafers are stacked between tungsten foils (0.5 mm thickness) andplaced in a graphite pressing die. To increase the speed of processingseveral sapphire/YAG:Ce ceramic/tungsten foil stacks can be stacked andprocessed simultaneously.

After evacuation of the HUP apparatus the temperature is first increasedto 1700° C. within 4 hrs without applying external pressure. Then auniaxial pressure of 300 bar is applied and kept constant for 2 hrs.After the dwell time the temperature is lowered to 1300° C. within 2 hrsby keeping the pressure constant. Finally, the system is cooled down toroom temperature within 6 hrs after releasing the pressure.

c) Post processing of sinterbonded sapphire-YAG:Ce wafers: Aftergrinding and polishing of the surfaces of the sinterbonded wafers, thesamples are annealed for 2 hrs at 1300° C. in air (heating rate: 300K/hr), then cooled down to room temperature within 12 hrs.

Having described the invention in detail, those skilled in the art willappreciate that, given the present disclosure, modifications may be madeto the invention without departing from the spirit of the inventiveconcept described herein. Therefore, it is not intended that the scopeof the invention be limited to the specific embodiments illustrated anddescribed.

What is being claimed is:
 1. A device comprising: a semiconductor lightemitting device; a ceramic layer comprising a first wavelengthconverting material disposed over the semiconductor light emittingdevice; a glass layer disposed directly on and contacting thesemiconductor light emitting device; and a second wavelength convertingmaterial disposed over the semiconductor light emitting device.
 2. Thedevice of claim 1 wherein the glass layer attaches the ceramic layer tothe semiconductor light emitting device.
 3. The device of claim 1wherein the glass layer is disposed between the ceramic layer and thesemiconductor light emitting device.
 4. The device of claim 1 whereinthe glass layer has an index of refraction greater than 1.8.
 5. Thedevice of claim 1 wherein the second wavelength converting material isdisposed in a transparent material.
 6. The device of claim 1 wherein thesecond wavelength converting material is a conformal layer.
 7. Thedevice of claim 1 wherein the ceramic layer is disposed between thesemiconductor light emitting device and the second wavelength convertingmaterial.
 8. The device of claim 1 wherein the ceramic layer is a solidagglomerate of phosphor particles substantially free of hinder material.9. The device of claim 1 wherein the glass layer is selected from thegroup consisting of high index optical glass, Schott glass SF59, Schottglass LaSF 3, Schott glass LaSF N18, and mixtures thereof.
 10. Thedevice of claim 1 wherein the glass layer is sol-gel glass.
 11. Thedevice of claim 10 wherein the sol-gel glass comprises one or morematerials selected from the group consisting of oxides of titanium,cerium, lead, gallium, bismuth, cadmium, zinc, barium, and aluminum. 12.The device of claim 1 wherein the semiconductor light emitting device isa III-nitride light emitting diode and the ceramic layer comprisesY₃Al₅O₁₂:Ce³⁺.